Add support to authenticate using GitHub App installation

[jcantosz]

Porting the work of @scottluskcis from migration audit: timrogers/gh-migration-audit#257, timrogers/gh-migration-audit#264

Summary

This PR adds in support for authentication using a GitHub App installation and also the use of the createAppAuth authStrategy that is part of auth-app.js. Specifically, new options have been added that allow specifying an app-id, private-key, and app-installation-id as option to the audit-all, audit-repos, and audit-repo commands. This will create an octokit instance that is authenticated as an installation, with automated installation token refresh as noted in Usage with Octokit.

This was necessary to take advantage of octokit automated installation token refresh. When running gh-migration-audit with a command like audit-all and it took longer than an hour the token would expire. Using an authStrategy of createAppAuth allows for the automatic token refresh and successful completion of the command for larger orgs.

See also notes available at Choosing between a GitHub App or a personal access token. This process needed to be run periodically and did not need to be tied to a user account but rather a GitHub App instance.

NOTE: Original PR was reviewed scottluskcis/gh-migration-audit#1, created another PR to have a squash commit and clean history.

Changes

• add new auth.ts file that handles creating the configuration passed to creation of an Octokit instance
  update the options for the commands to optionally allow specification of the app-id, private-key, and app-installation-id
• add tests for the auth.ts file added
• add samples to the README for the new options and explanation in Authentication section
• update octokit.ts to accept a string for token as it did before or an AuthConfig object, backwards compatible
  This pull request introduces significant changes to the authentication mechanism used in the repository audit tool. The most important changes include adding support for GitHub App installation authentication, updating the command options to accommodate the new authentication method, and refactoring the code to handle both token and installation authentication seamlessly.

Documentation
Updated README describing changes

Usage
New options displayed in --help, note addition of --app-installation-id, --app-id, and --private-key. Also note environment variable display for these and also for --access-token

Closes: #455